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Here are some considerations when working with the firewall on a Lancair 3x0:
1. FAA certification to Part 23 would require 3 layers of fiberfrax. This is
overkill and also too thick to avoid distorting the SS firewall at each
accesory attachment bolt. Two layers is safe and sufficiently thick.
2. For the engine mount, I used an area washer between the firewall and the
SS firewall and cut the fiberfrax out around the washer. Note that after 80
flight hours I was able to re-torque the mount bolts quite a bit (this
eliminated some airframe vibration). If I had left compressible fiberfrax at
the mount locations, would I have had to re-torque more?
3. Where possible, I used nut-plates on the cockpit side of the firewall
(especially up in the header tank area) so that parts mounted with bolts could
be removed without removing the forward deck.
4. I cut a firewall out of hardware store "window pane " plexiglass and
drilled the holes in it for the engine mount (just like the the wood
firewall). I also had my engine mounted on the engine mount and hung from the
engine lift lug (not attached to the airplane). With this setup, I could put
the clear plastic firewall on the engine mount and see where I could mount
some accessory for fit to the engine. This was marked with a marking pen and
holes drilled in the plastic. The plastic could then be moved to the wood
firewall for fit check and also used as a "see thru" template for drilling the
SS firewall.
5. SS is not that hard to drill with sharp drills. However, smaller holes
(up to an inch) were started with a "bullet point" 1/8 inch drill (eliminates
drift) and finished with a "step" drill to the desired size. Step drills are
fabulous for drilling thin material since they don't "grab". All holes were
deburred. Large holes can be nibbled and filed.
6. The outside edge of the SS firewall can be beautifully finished by rolling
in a 10 to 20 degree 1/4 inch bevel with a "lightening hole stiffening tool".
This also stiffens the outside edge of the SS firewall.
7. If Camloc spring-loaded fasteners are being used to hold the cowling to
the fuselage, remember that once the receivers are riveted to the fuselage
flanges, the SS firewall would not be mounted or removed without severely
bending it. There are at least 3 solutions:
a. (normal) Do not permanently attach the receivers to the lower fuselage
flanges until you have mount the SS firewall for the final time.
b. (weird) Provide for vertically splitting, then overlapping the SS firewall
from above the nose gear opening thru the "prop governor" opening and straight
up. If no governor opening is needed, don't do this.
c. (anal, my method) Attach 1.5 inch wide strips of aluminum to the lower
fuselage cowl flanges. Put nutplates on the aliminum strips and flathead
screws thru the flange. These screws are placed between the places that
Camloc receivers will be located. Mount the receivers to these aluminum
strips. If one wishes to remove or replace the SS firewall, one unscrews and
removes the aluminum strip, then slips the SS firewall down and out.
Scott Krueger
N92EX
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